Population geneticist and evolutionary theorist Sewall Wright coined the term "genetic drift" to describe the random events that change the frequency of various alleles in a population. He used mathematics to explain the action of genes in evolution, showing how natural selection, mutation, and other evolutionary pressures affect the frequencies of alleles (viable DNA coding that occupies a specific position on a chromosome) and genotypes (the genetic makeup of an organism).

Introduced in his 1931 paper "Evolution in Mendelian Populations," Wright's notion of "shifting balance" holds that a species' population is subject to different evolutionary pressures as groups become partially isolated, geographically, from others of their species, and that as one of these isolated groups arrives at a state that is better adapted to its environment through random gene-frequency drift, migrants from that group eventually travel and bring that evolutionary upgrade the species at large. This concept was disputed over several decades by Ronald Fisher, who argued that the random gene-frequency drift was of minimal importance, and that instead large populations with large-scale selection is the most effective driver in evolution.

Wright spent the first decade of his career at the Department of Agriculture, where he perfected the inbreeding coefficient, a mathematical algorithm for determining the effect of inbreeding on heterozygosity (the presence of different alleles at one or more loci on homologous chromosomes).

Author of books:Systems of Mating and Other Papers (1949)Genetic and Biometric Foundations (1968)Evolution and the Genetics of Populations (1968-78, four volumes)Experimental Results and Evolutionary Deductions (1977)Variability Within and Among Natural Populations (1978)Evolution: Selected Papers (1986)